A number of signal processing schemes for interferometric fiber sensors which make use of frequency modulation (FM) of a semiconductor laser light source are known in the art. These approaches are designed to overcome the classical sensitivity null and direction of phase change ambiguity problems in interferometers, at which points the interferometer is not useful for measuring small changes in the relative phase shift .omega.. In particular, they are applicable to unbalanced Michelson and Mach-Zehnder interferometers and to low-finesse Fabry-Perot interferometers. The commonly used phase generated carrier approach employs a sinusoidal FM modulating waveform at radian frequency .omega. with mixing, differentiation, cross-multiplication, subtraction, and integration to recover .phi.. This approach is described by Dandridge et al. in IEEE Journal of Quantum Electronics, vol. QE-18, pages 1647-1653, Oct. 1982. An alternative method uses a sawtooth FM waveform (linear chirp) for the laser output, with a phase detector for demodulation. This method is described by Jackson et al. in Electronics Letters, vol. 18, pp. 1081-1083, Dec. 9, 1982.
The FM signal processing schemes described above use analog electronic circuits. As the long-term trend towards less expensive digital signal processing (DSP) integrated circuits continues, their use becomes less cost-effective. In one such case, the phase generated carrier approach was implemented with a combination of a preprocessor providing analog sin .phi. and cos .phi. inputs to a digital processor which computed .phi.. This approach is described by Bush and Sherman in SPIE, vol. 1795, pp. 412-420, 1992. In another case, a square current pulse produced frequency modulation of a laser, the sensor output was sampled and digitized at two different times during the pulse, and .phi. was determined using a look-up table. This is described in by Yeh et al. in SPIE, vol. 1584, pp. 72-78, 1991.
Thus, by using a continuous wave, constant frequency light source, it is difficult to measure small phase changes in an interferometer with a high degree of precision. It has become desirable to overcome the problem of sensitivity nulls, ambiguities in direction of phase changes, and nonlinear dependence of optical output on phase shift by employing digital processing circuits and methods.